Ground Investigation: Delving into details

Additional ground investigation has proved essential for one section of the Thames Tideway scheme.

Work on constructing the 25km Thames Tideway sewer scheme has now broken ground at a number of sites on its route from Fulham in the west of London to Abbey Mills Pumping Station in the east (see box). Although the scheme has been many years in the planning with reams of knowledge acquired in the process, a final stage of ground investigation work has proven essential to guiding construction techniques on one section.

Ground Investigation specialist ESG was brought in by the CVB joint venture, which is formed from Costain, Vinci Construction Grands Projets and Bachy Soletanche, to provide more detailed understanding of the ground conditions on the eastern section of the Tideway scheme. CVB’s contract for the tunnel between Chambers Wharf in Bermondsey and Abbey Mills involves construction of five deep shafts in Chalk, including two tunnel drive shafts at Chambers Wharf and Greenwich.

ESG’s remit was to carry out detailed geotechnical assessment of five sites and assist in gathering data to identify the optimum construction solution for the Chambers Wharf shaft.

The Chambers Wharf shaft is a key part of the Tideway project. As well as providing a point of access to the excavation work for the main tunnel, it will also be the construction site for part of the sewer’s ventilation system and a new river wall will be built there to replace the old flood defences.

A number of temporary structures, including a cofferdam, will also be constructed there during the work to protect the site from the tide during the tunnelling.

The ground conditions at the site are as complex as the construction that is planned there. Part of the site is located on recently cleared brownfield land, while the other part covers the adjacent river foreshore, so had the potential to feature a wide range of soil strata and surface deposit types, as well as buried structures.

ESG was commissioned to undertake core drilling services, insitu testing and laboratory analysis to provide clear characterisation and a robust ground model for CVB and its designer Mott MacDonald.

One of the key issues that CVB wanted to gain a greater understanding of was the potential for fissures deep in the Chalk in order to inform its design for grouting of the Chalk.

ESG undertook 100m deep boreholes in the Chalk and all cores were logged on site to check for the size and number of fissures. These boreholes were also used to carry out high pressure dilatometer (HPD) pressuremeter testing, packer testing and down hole geophysical analysis to determine the strength and resilience of the rock in situ.

Findings of the core analysis were shared with CVB and Mott MacDonald in real time so that all three organisations could collaborate to devise and undertake additional tests or deploy new testing methodologies to provide CVB with the required understanding about the nature of the ground beneath the site.

Driller’s logs were recorded in ESG’s proprietary geotechnical data application Silas, which allowed results to be relayed to ESG staff working off-site in real time to enable further analysis to be undertaken if needed.

According to CVB lead geologist Matt Bellhouse, the close working relationship helped to ensure the team’s flexibility and versatility when developing additional tests to study newly-identified anomalies.

“ESG worked closely with our team from the very beginning of the project,” says Bellhouse. “It was great to see how well the logging of the site’s stratigraphy had been carried out, with the key fossils identified to support the team’s interpretations.”

ESG assistant operations manager Hannah Dwane adds: “Collaborating with CVB and its partners, we were able to devise the most appropriate testing methodologies to provide accurate data on an extremely busy and cramped site, without impacting on the project schedule. As a result, CVB had the information it required to undertake its shaft design, while our work was carried out efficiently and on time.”

With the detailed ground investigation completed and additional knowledge secured, CVB has now got construction of the shaft at Chambers Wharf underway with added confidence in the design approach.

The Tideway lowdown

The tunnel generally follows the path of the river from the start of the system in Fulham in west London to Limehouse in east London

Construction of the £4.2bn Thames Tideway Tunnel will result in a cleaner River Thames by minimising combined sewer overflow discharge into the river during periods of high rainfall. In the past this used to happen once or twice a year but now can occur on a weekly basis and a storm is no longer needed to trigger an overflow with up to 40M.m3 in overflows recorded each year.

The 25km long inception, storage and transfer sewer Thames Tideway Tunnel, which will run up to 66m below the River Thames, is set to cut overflows to around 2.3M.m3 a year.

The tunnel generally follows the path of the river from the start of the system in Fulham in west London to Limehouse in east London. From Limehouse the tunnel cuts inland towards Abbey Mills Pumping Station where it will connect with the Lee Tunnel, which will transfer sewage to the Beckton Sewage Treatment Works.

Construction of the tunnel is split into three contracts. The western section is being constructed by a joint venture of Balfour Beatty, Bam Nuttal and Morgan Sindall. The central section – the longest tunnel drive of the three – is being built by Laing O’Rourke and Ferrovial Agroman. The western section is being delivered by the CVB joint venture, which is formed from Costain, Vinci Construction Grands Projets and Bachy Soletanche.

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